U.S. Pat. No. 4,695,430 is directed to an apparatus for automatically performing analytical testing of biological fluids. Individual samples to be tested are introduced into sample cells, where they are injected with one or more reagents and the resulting reactions optically observed. The apparatus comprises a memory for storing a number of different test protocols. All of the tests are performed on the test samples within the sample cells.
During the course of analytical testing, the sample cells are transported to various parts of the apparatus. For example, in a typical test, a sample cell may be loaded with sample fluid at one point, transported to another point to receive reagent, and then transported to a third point to be optically scanned.
U.S. Pat. No. 4,695,430 discloses two different types of sample cell. One type is designed to receive biological fluids that do not contain non-fluid components; the other is for receiving biological fluids that contain non-fluid components. The two types of sample cells are identical except that the second type is adapted to function with a filtering mechanism in the apparatus wherein the non-fluid components are removed, leaving only fluid to be tested. The improvement of the present invention is directed to the second type of sample cell.
Both types of sample cell comprise two parts, a body and a slide, slidable relative to each other. The slide comprises reservoirs for holding the sample of biological fluid to be tested and into which test reagents are added. The reservoirs and the slide may be moved relative to the body to one of two positions, depending on whether the sample of biological fluid is being introduced and the reagents are being injected and the reaction observed. Movement from the first position (loading) to the second position (analysis) is called "indexing".
The second type of sample cell is particularly well-suited for receiving whole blood for testing. Testing is done on plasma, however, and not whole blood.
When extracting plasma from whole blood it is largely unpredictable how much plasma will be obtained from a given quantity of whole blood. The hemocrit (or hematocrit) level of a whole blood sample-the percentage of whole blood volume occupied by red cell after centrifugation--may be anywhere from 15% to 70% among samples typically tested. Consequently, it is likely that a given quantity of whole blood will yield an amount of plasma in excess of the capacity of the reservoirs in the slide. When this happens, the excess plasma will tend to overflow and seep out into the gap between the body and the slide. Then, when the sample cell is indexed, the excess plasma will smear between the body and the slide, and surface tension may cause plasma to be drawn out of the reservoir, thus unpredictably varying the volume of plasma actually being tested, rendering analytical results meaningless.
It is an object of the present invention to prevent smearing of plasma between the body and the slide, by shearing the excess plasma from the gap between the body and the slide.